The present invention relates generally to gas turbine engines, and, more specifically, to turbine nozzles therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases which flow downstream through several turbine stages that extract energy therefrom. A high pressure turbine includes a turbine nozzle at the outlet of the combustor which channels the hot combustion gases to the high pressure or first turbine stage of rotor blades extending radially outwardly from a supporting disk.
The turbine nozzle includes hollow vanes through which air bled from the compressor is channeled for providing internal cooling of the vanes for protection against the hot combustion gases. The vanes typically include rows of film cooling holes through the airfoil surfaces thereof for discharging the cooling air in protective film cooling layers to provide additional thermal insulation against the hot combustion gases which flow thereover.
In view of the high operating temperature of the high pressure turbine nozzle, it is typically formed in arcuate segments having two vanes supported in corresponding outer and inner bands, formed in corresponding arcuate segments. The individual bands and vanes are separately cast from high strength superalloy materials for maximizing their strength at the high temperature environment of the turbine nozzle.
The four basic parts of each nozzle segment are then assembled together by inserting corresponding hubs at opposite ends of the vanes in complementary seats in the bands, and then brazing together the vanes and bands to provided a fixed, integral assembly thereof. The nozzle segments are then assembled together in a complete annular ring with suitable seals therebetween and mounted in the engine at the outlet of the combustor.
Film cooling of the nozzle vanes is used to ensure nozzle durability and corresponding long life. The nozzle bands are typically cooled from their outboard surfaces, and typically include film cooling holes extending therethrough for film cooling their inboard surfaces. The nozzle bands define the radially outer and inner flowpath boundaries for the hot combustion gases along which the gases have a relatively low velocity boundary layer, with the velocity thereof increasing to the midspan of the vanes.
This type of nozzle has been used in commercial service in this country for many years with long life exceeding about 20,000 hours of operation or about 20,000 cycles of operation. Examination of these high cycle turbine nozzles has uncovered a problem limiting additional life of the nozzles beyond these high cycles. The nozzle bands and vanes have proven to be highly durable and have sufficient integrity for further cycles of life. However, the braze joints between the vanes and bands have deteriorated and require the replacement, or significant repair, of the turbine nozzle.
Accordingly, it is desired to improve the turbine nozzle configuration for further increasing nozzle durability and life by reducing braze degradation.
A turbine nozzle includes a plurality of airfoils and integral hubs brazed into corresponding seats of outer and inner bands. The hubs have brazeless fillets blending the bands to the airfoils.